It is well known in the art that images can be reproduced by a photographic technique using a light-sensitive composition comprising a polymerizable compound having an ethylenically unsaturated bond, a photopolymerization initiator, and sometimes other components, such as an appropriate binder having a film forming property and a heat polymerization inhibitor. In this technique, a light-sensitive layer composed of the above-described light-sensitive composition is exposed to light through a mask of a desired pattern, thereby curing and insolubilizing the light-sensitive layer, and the unexposed areas are removed by an appropriate solvent to obtain a cured image of the desired pattern, as described, e.g., in U.S. Pat. Nos. 2,927,022, 2,902,356 and 3,870,524. It is very natural that this type of light-sensitive composition is useful for the production of lithographic printing plates or photoresists.
Use of a photopolymerization initiator in combination with the polymerizable compound has hitherto been proposed in order to increase light sensitivity of the light-sensitive composition since only the polymerizable compound having an ethylenically unsaturated bond is not sufficient to impart satisfactory light sensitivity to the composition. Such a photopolymerization initiator includes benzyl, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, 2-ethylanthraquinone, and the like.
However, use of these photopolymerization initiators cannot raise the curing rate of the photopolymerizable composition, thus requiring a longer time for imagewise exposure for image formation. Therefore, when a fine image is desired, even a slight vibration in operation leads to poor quality of images, and satisfactorily fine images cannot be obtained. Further, the quantity of energy emitted from a light source for exposure must be increased, so that considerations must be given to the evolution of a large quantity of heat associated with the increased energy. Furthermore, the light-sensitive layer is apt to be deformed or degraded due to such heat.